The present invention relates to a means for reducing the resonance frequency of the vibrating plate of an electro-magnetic acoustic transducer (hereafter referred to as "transducer").
A construction of a conventional transducer is shown in FIG. 1. An exciting portion is composed of a permanent magnet 5 and an exciting coil 6 mounted on back plate 7 including a magnetic core 7a and housed in a vibrating plate supporting member 3. The periphery of a vibrating plate 2 arranged opposite to the exciting portion and formed in one body with a yoke 1 by welding or the like is supported by the vibrating plate supporting member 3 having the vibrating plate support diameter D. In the conventional construction, the vibrating plate support diameter D is increased or the volume of the yoke 1 is enlarged in order to reduce the resonance frequency without reducing the sound pressure when the resonance frequency of the vibrating plate 2 is high. However, the above means is disadvantageous since the shape of the transducer is enlarged and the thickness thereof is increased.
It is an object of the present invention to obtain a transducer provided with a sufficient sound pressure by reducing the resonance frequency stably without enlarging the shape of the transducer.
In accordance with the invention, an air hole is provided which opens into the space defined by the back plate, vibrating plate and vibrating plate support member and the air hole is sufficiently large to reduce the resonant frequency of the vibrating plate while maintaining the sound pressure in such space.